460 



Mr. W. B. Sumpner on the Variati 



A. 



W 



33. 



p. 



e- 



•0366 



•184 



665 



780 



0-852 



•0275 



•156 



424 



661 



•640 



•0220 



•141 



306 



597 



•512 



•0147 



•112 



163 



474 



'342 



■0110 



•091 



99 



385 



■256 



•0055 



•071 



38-6 



301 



•128 



■0029 



•062 



17-8 



263 



•068 



•0044 



•066 



28-7 



279 



•102 



•0100 



•091 



89-8 



385 



•233 



•0200 



•128 



253 



542 



■465 



•0400 



•189 



746 



801 



•931 



7. As the magnetic circuit in this case was entirely composed 

 of iron, it was easy to reduce the observations to absolute 

 measure. The values of the induction 33. the magnetic 

 force .£), and the magnetic permeability //. were calculated in 

 C.G.S. units from the formulae 



CO 1()8 T A a 4 ™ A 10°' T 



<8 = - g L 2 A, $ = Wl A, ^j-rgL,, 



where A was the current in amperes, L 2 the coefficient of 

 self-induction in secohms, S and I the mean values of the 

 sectional area and length of the magnetic circuit in centi- 

 metres, and n the number of turns. 



The iron core was half an inch in diameter and 14 inches 

 long, and the distance between the poles was 3 inches. The 

 number of turns was 800. Whence 



Z=17x 2-54 = 43-2, S= |(?|- 4 J = l-27, ti=800. 



33 = 98,700 L 2 A, £ = 23-3 A, p = 4,240 L 2 . 



The numbers obtained very approximately fulfil the following 



relations: — T A A ^ , n . 



L 2 =0-0o + 3-9 A, 



^=210 + 720 £, 



S3 = 210 £ + 720 £ 2 . 



The values were obtained for reversals of magnetizing force 

 whose semiamplitude «f) was greater than 0*06 and less than 

 0-9 C.G.S. units. Beyond these limits the value of S3 pro- 

 bably differs from that given by the above relation. By 

 employing the secohmmeter of Professors Ayrton and Perry, 

 it would have been possible to obtain values of L 2 for much 



